Will she … won't she. UK forecasters remain uncertain about the after-effects of hurricane Bertha – pictured off the US eastern seaboard this week. Photograph: Getty Images

Forecasters have warned that parts of Britain can expect almost a month's rainfall in the space of a few hours and strong winds, especially in the south, on Sunday, with the risk of flooding as the remains of hurricane Bertha reach this side of the Atlantic.

The Met Office has issued a severe weather warning for rain for Sunday covering Wales and southern England, bringing at least a temporary end to a glorious summer. "There is the potential for widespread rainfall totals of more than 50mm and coastal gusts of over 60mph, along with large waves," said the Met. "However, the system may pass harmlessly to the south of the country or spread heavy rain even further north, and the public are advised to keep up to date with warnings."

The average rainfall for the whole of August in England is 69mm.

The Met has been tracking Bertha for the past few days, trying to determine its strength and direction. It is a notoriously difficult task and the forecasts have to be adjusted according to the data. "The transition from a tropical to an extra-tropical feature is a particularly hard one to forecast with confidence, and computer models continue to differ in the location and intensity of the resulting depression, which is expected to pass over, or close to, the UK from early on Sunday," the Met said.

On Wednesday, forecasters thought Britain was unlikely to suffer serious after-effects from the hurricane, which battered the Caribbean islands with gusts of more than 90mph, leaving thousands of homes without power. They said it was debatable whether what would arrive was even ex-Bertha as the storm weakened to such an extent as it crossed the Atlantic.

On Thursday, however, they changed tack, warning of heavy rain and a possible risk of flooding.

"On Sunday and Monday, a combination of high spring tides and strong westerly winds brings a possible risk of flooding to the south-west coast of England and along the Severn estuary, while heavy rain may also lead to localised surface water flooding in parts of southern and central parts of England," said Craig Woolhouse, flood risk manager at the Environment Agency.

"However, the forecast remains uncertain so we advise people to regularly check the flood risk situation over the next 48 hours."

On Friday, parts of Britain will see some heavy rain that is not linked to Bertha.

Bertha, the second hurricane of the 2014 Atlantic hurricane season, began as a tropical storm on 31 July about 275 miles east-southeast of Barbados. It brought some welcome rainfall to drought-suffering parts of Puerto Rico, the Virgin Islands, and eastern Hispaniola.

Much of Puerto Rico picked up at least 25mm of rain, with some interior mountainous locations estimated to have topped 254mm, with reports of partial road collapses.

Bertha's impact on the US took the form of high surf and "rip" currents that move directly away from the shore. One man drowned due to rip currents Florida.